The invention relates to a method for producing cannon and gun tubes of 105 to 120 mm caliber and greater.
The standard material for these products is the steel 35NiCrMoV 12-5, Material No. 1.6959, described in the Stahl-Eisen-Liste [Steel-Iron List] of the publishers Stahleisen, Dxc3xcsseldorf, and in the material data sheet xe2x80x9cRohrstahl fxc3xcr schwere Geschxc3xctzexe2x80x9d [Steel for Tubes of Heavy Guns] of the BWB [German Federal Office of Armaments Technology and Procurement]. The production process for cannon tube blanks comprises the work steps of open smelting, pouring of raw ingots into suitable casting die formats, forging of the cannon tube blanks into exterior rough shapes, annealing the forged pieces, pre-working on a lathe and pre-boring of the parts, heat treatment of the hollow parts (hardening and tempering to the requested strength), measuring the distortion (out of true, i. e. the maximum deviation from the straight line of the longitudinal axis in respect to the bearings at the tube ends) due to hardening, mechanical straightening (trueing) and subsequent annealing to approximately 30xc2x0 C. below the tempering temperature, performance of quality checks and finishing of the cannon tube blanks to the requested dimensions.
The work step of straightening to obtain trueing after the heat-treating process represents a qualitative problem in the course of the conventional production process, because by this straightening step the straightness of the bore is not achieved and internal ductile strains are induced. Further, after the straightening step it is not possible to straighten a distorted, pre-bored bore in the course of the subsequent boring to the requested size, and remnants of internal stresses still remain in the material in spite of stress-relieving annealing after straightening. It was shown under actual conditions that a) bores out of true and internal strains lead to distortions during the finishing of the tubes, which can only partly be compensated by additional straightening operations, b) waste can be created in the course of processing by dimensional discrepancies on account of the distortions, and c) the firing accuracy (system errors) can become worse on account of deviations from the straightness of the bore and because internal stresses can be released during firing.
As shown by tests in connection with the invention, three main causes are responsible for the distortion during hardening:
1. There can be an asymmetric temperature distribution in the tube blank. It is caused by uneven heating, uneven furnace temperatures or uneven heat distribution. This can be overcome by homogeneous heating and precise temperature distribution in the furnace chamberxe2x80x94a check can be performed by means of thermal elements on the piece. Rotation of the tubes during the entire heat treatment can also aid in this.
2. There may occur a mechanical distortion during heating and austeniting to the hardening temperature. It is created by bending moments during heating in a horizontal position and even in a vertical position if it is a rigid suspension. Such bending moments are the result of inherent weight or horizontal movement during hardening. The distortion can be prevented by suspended (vertical) heat-treating of the tubes by means of suspension from gimbals, so that no bending moments can occur in the tubes at the suspended end in the case of a horizontal movement.
3. A further reason for distortion can be asymmetric transformation strains. In the course of hardening the pre-bored tube blanks the exterior surface as well as the bore are cooled as evenly as possible by the application of water. When the martensitic start temperature of approximately 350xc2x0 C. has been reached in the material, the austenitic structure begins to be transformed into the martensitic hardening structure. With low distortion hardening, transformation takes place over the entire circumference from the outside (outer surface) toward the inside, and from the inside (bore) toward the outside, until the transformation fronts meet and the entire tube cross section has been hardened. If, because of production, the normal segregation is asymmetric, the transformation processes starting from the bore inevitably start at different times in accordance with the different local analysis situation. This leads to an asymmetric distribution of the transformation strains over the tube cross section and therefore to hardening distortion.
It has been shown in the course of the actual production of cannon tubes that, although the start of transformation at the outer surface takes place symmetrically in the circumferential direction, it does not always do so in the area of the bore. The reason for this primarily lies in the fact that often there is an asymmetry of the bore in relation to the axis of the ingot or in relation to the solidification symmetry of the ingot. FIG. 1 shows a tube in the center position of the raw ingot and its segregation symmetry which will lead to relatively slight distortion when the hollow tube is heat-treated. In contrast, the eccentric position of the tube in relation to the raw ingot shown in FIG. 2 will result in relatively greater distortion.
It is not always possible to avoid an eccentricity of the bore in relation to the former ingot axis because of uneven material flow, which often cannot be prevented, as well as dimensional tolerances (offset) during forging. In consequence, there are asymmetric analysis concentrations, resulting from segregation, in the surface of the bore which cause uneven transformation strains in the interior of the tube leading to distortions.
It is an object of the invention to avoid the inaccuracies mentioned and the production difficulties connected therewith.
The new method proposed for the solution of the above problems is characterized in that the tubes for heavy guns heavy guns in the caliber range of 105 mm and greater are made from heat-treatable steel consisting in wt.-% of 0.20 to 0.50% carbon, max. 1.0% silicon, max. 1.0% manganese, max. 0.03% phosphorus, max. 0.03% sulfur, max. 0.1% aluminum, max. 4% nickel, max. 2% chromium, max. 1% molybdenum, max. 0.5% vanadium, and the remainder of iron and the customary impurities, wherein forgings of open-smelted cast ingots are preworked on a lathe on the outside and the solid blanks obtained in this way are hardened and tempered, subsequently drilled and then finished.